how do you determine keywords?

I start from what people actually type, not what I'd call it. Search Console's Performance report shows the queries you already get impressions for, which is the best list you'll ever have. For new topics, type the main term into Google and read the autocomplete and the "People also ask" box. Then google each candidate and look at what ranks. If it's all product pages and you were going to write a blog post, that keyword isn't for that page, whatever the volume says.